C&C and MWFRS Wind

[VenkatKannan]

Hi All,
what is different between C&C & MWFRS wind, in a structure what are the members need to be design with C&C wind? and what are the members need to be design with MWFRS Wind?
Thanks In advance

 

[TehMightyEngineer]

MWFRS loads are used to design the entire structural lateral force resisting system (frames, shear walls, diaphragms, foundations, etc.) basically anything that doesn't see load directly.

C&C wind loads are used to design the components that directly see and distribute wind loads. Wall and roof sheathing, windows, parapets, girts, purlins, fasteners, etc.

If the wind blows on it, use C&C loads. If it distributes wind loads, use MWFRS loads. Sometimes items have to be designed for both.

Don't forget minimum wind load pressures for C&C and the minimum wind load cases for MWFRS design.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[VenkatKannan]

i got the clear explanation, thanks

 

[Trillers]

TehMightyEngineer, we recently had a C&C issue come up regarding downspouts. If the 4" aluminum downspout is flush against the building which factors apply?
In this case I just assumed full frontal assault from the wind and designed the anchorage for that since shear would be the concern. The wind force we applied parallel to the building and assumed a 4" wide component area.
We Selected this method because there's just didn't seem to be any applicable C&C design parameter s in either ASCE 7 OR THE IBC

 

[TehMightyEngineer]

The usual statement about the code can't replace engineering judgement would apply there in my mind. I agree and would also make a realistic assumption for the practical loads the downspout should be designed for. I'd agree that a shear load parallel to the building using conservative C&C loads makes the most sense.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[JLNJ]

A downspout that is on an outside corner would be exposed to some pretty big forces compared to something in the middle of the building. Such forces might be beyond the code to estimate.

 

[charliealphabravo]

I haven't seen the 2016 ASCE-7, but if the definitions for C&C and MWFRS have not changed then neither has my opinion. I have pasted my response to a previous post.

A structural element should be designed for MWFRS pressures if it is part of the MWFR system and for C&C pressures if it is not. This determination is made using the definitions of MWFRS and C&C given in the Standard (ASCE 7). Note that these definitions do not mention anything about 700sf or Effective Wind Area. The language about 700sf is an exception that allows C&C to be designed using MWFRS pressures if the Effective Wind Area of the C&C is 700sf or more. This provision should not be interpreted to mean that no elements in the MWFRS will have wind areas less than 700sf. In fact, Effective Wind Area is not really relevant to MWFRS analysis. The tabulated pressures for MWFRS are not a function of Effective Wind Area; nor are Effective Wind Areas the same as tributary areas.

Also note that, to be precise, the definitions of MWFRS and C&C are mutually exclusive. That is to say, the way the definitions are written, an element cannot be both MWFRS and C&C. I am not aware of any provision in the Standard that speaks to the design of structural elements to both MWFRS and C&C pressures; or any language in the Standard that requires small MWFRS to be designed for C&C pressures. You can find language that recommends this in Kishor’s Guide to ASCE7 and you might possibly infer it from language in the Standard Commentary. It is left to your professional engineering judgment to determine if MWFRS pressures or C&C pressures or both must be used. It is left to your professional engineering judgment to determine whether a structural element is C&C or part of the MWFRS system; notwithstanding the lists in the Commentary that would seem to imply that a certain structural element (a truss for example) is both C&C and MWFRS. This cannot be determined without knowing how the structural element is loaded which is why this language is in the Commentary and not in the Standard.
​

The standard was developed before my time but I suspect that the MWFRS and C&C terms were originally developed for large commercial buildings and PEMBs where the distinction between cladding and MWFRS is typically distinct. It gets more difficult (in residential construction for example) where structural elements such as masonry walls and exterior sheathing may serve both purposes.

 

[Lomarandil]

Depending on the structure, it may be best to design the downspout connections for the weight of a child trying to climb it

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The name is a long story -- just call me Lo.